I want to opt-out of spending limits. Will it apply instantly?

No, it will come into effect after a delay from the time of request. See the table below:

DAILY LIMIT: ACTIVATION AFTER 24 HOURS FROM REQUEST.

WEEKLY LIMIT: IF REQUEST GIVEN ON LAST DAY OF WEEK, ACTIVATION ON END OF NEXT WEEK (I.E. SATURDAY MIDNIGHT). IF REQUEST GIVEN ON ANY OTHER DAY OF THE WEEK, ACTIVATION ON START OF NEXT WEEK (I.E. SATURDAY MIDNIGHT)

MONTHLY LIMIT: IF REQUEST GIVEN ON LAST DAY OF MONTH, ACTIVATION ON END OF NEXT MONTH. IF REQUEST GIVEN ON ANY OTHER DAY OF MONTH, ACTIVATION ON START OF NEXT MONTH.